This invention relates to the production of a D.C. analog signal proportional to the peak amplitude of an A.C. signal, which peak amplitude is proportional to the frequency of the A.C. signal, and more particularly to a method of producing from one or more A.C. signals a D.C. analog signal that may be filtered with a filter having a relatively low time-constant.
Although for illustration of a particular application of the invention, the source of the alternating current signals described as being a rotary magnetic encoder, it will be understood that the invention is applicable, as well, to any case where an analog signal is derived from one or more alternating current signals.
A rotary magnetic encoder may include a cylindrical rotating magnetic member on the outer circumference of which is a continuous series of N-S magnetic poles. A stationary member holds one or more detectors so disposed that, as the rotating member rotates, the flux at successive magnetic poles links the poles with the detector and the detector produces a characteristic sine wave output. Each full cycle of the sine wave represents the movement of one N-S pair of poles past the detector. It is common to employ two detectors on the stationary member, displaced 90 electrical degrees from one another, with respect to the magnetic poles to double the number of total pulses per revolution for increased resolution. Thus, two essentially identical sine waves in quadrature with one another are produced with identical frequencies proportional to the speed of the rotating member. These two sine waves may then be rectified and filtered to produce an analog signal proportional to the frequency of the sine waves.
The processing of the sine waves may, in some applications, result in undesirably high ripple in the analog signal and high time-lag in changes of that signal behind changes in the peak amplitude, consequent upon changes in frequency, of the A.C. signal. These two characteristics are related by the nature of the rectifying and filtering steps because rectification of the two sine waves produces a waveform with only twice the frequency of the primary waves, with a relatively large amount of ripple. The low frequency of the ripple voltage requires a filter with a high time-constant and the large amount of ripple requires a high degree of filtering. This results in an analog signal which responds to changes in the peak amplitude of the A.C. signal with a time lag to an undesirable degree for some control applications.
Accordingly, an object of the present invention is to provide a method of producing a D.C. analog signal which is proportional to the peak amplitude of one or more identical A.C. signals, which is relatively free of ripple, and which responds to changes in such peak amplitude with a minimum amount of lag time.
The present invention overcomes some of the limitations of prior art by increasing the frequency of the ripple voltage of the rectified waveform so that filtering may be accomplished with a high-frequency filter having a low time-constant. In one embodiment of the invention, the means by which the frequency is increased further reduces the degree of filtering required, since the greater the ripple frequency, the lower will be the amplitude of the ripple.
It will be understood that, in accordance with conventional nomenclature, whenever "sine wave" or the equivalent is used herein, it means an electrical alternating current signal having a sinusoidal waveform. It will also be understood that the signal may not be a pure "sine wave", but is a periodic signal having sinusoidal characteristics.